«I am interested in the animals» suggests a group of animals (my animals, the animals that inhabit this island, the animals that he paints...) . while the french sentence, «je suis intéressé aux animaux» refers to all animals. Otherwise it would be «je suis intéresé à des (ou certains, quelques) animaux.
I think that «i am interested in animals» would be a good translation
Je m'intéresse aux animaux can be translated correctly to either I'm interested in animals or I'm interested in the animals. To say je m'intéresse à des animaux one would say I'm interested in some animals. Likewise, je m'intéresse à certains animaux= I'm interested in certain animals ; je m'intéresse à quelques animaux = I'm interested in some / a few animals. (Edited from standard to pronominal forms)
the sentences you get are picked randomly among a set. The random part means some will pop up more than others, and if you stick to one particular lesson (which can easily happen if you use the "practice" feature for example), you'll keep drawing from the same set. Moreover, those sets grow in size as you progress, so until you go to more advanced lessons it might feel like you're getting the same stuff often.
Hang in there, and happy learning!